Field of the Invention and Description of the Related Art
This invention relates to a method and an apparatus for detecting and measuring the refractive index of a substrate for an optical disc, which can precisely detect and measure the main axis of an ellipsoid of the refractive index (i.e., an index ellipsoid).
Recently, there has come to be noted an optical information recording and reproducing apparatus whereby information can be recorded at a high density in an optical recording medium by condensing light beams and projecting them onto this recording medium and the recorded information written into the recording medium can be read out (reproduced) at a high speed by receiving with a light detector the light returning from this recording medium.
The above mentioned recording medium is divided into a write-once type in which information cannot be erased after being once written, and a magneto-optical type in which a magnetic layer is formed to make information erasable after once written by utilizing the optical Kerr Effect.
A substrate for the above magneto-optical recording medium may be formed of acrylate resin such as PMMA (polymethyl-methacrylate). This acrylate resin is high in optical characteristics but has a defect in that the hygroscopicity is so high as to deflect the recording medium surface.
Therefore, it is desired to use for the substrate a material which is hard to deflect, high in form stability and high in mechanical strength. It is also desired for this substrate to have good optical characteristics.
As disclosed in U.S. Pat. No. 4,792,227, since light beams are condensed in a substrate of the magneto-optical or other type, double refraction produced in the substrate is responsible for lowering the S/N ratio of a reproduced signal.
The above mentioned substrate for the recording medium is injection-molded in many cases. In these cases, the substrate is more easily susceptible to distortion which will often exhibit double refraction.
The substance showing optical anisotropy generally has the three main refractive indices N.sub.1, N.sub.2, N.sub.3 (or n.sub.1, n.sub.2, n.sub.3) in different directions orthogonal to one another. The ellipsoid defined by these three main axes is an ellipsoid of the refractive index.
The degree of the above mentioned double refraction can easily be known from the status of the index ellipsoid for the substance.
Assuming that a ray (light beam) propagates in the direction of S, as shown in FIG. 1, the plane of the index ellipsoid IE sectioned by a plane perpendicular to S and passing the center O of the index ellipsoid IE is generally an ellipse. The difference between the long (major) axis and the short (minor) axis of this ellipse corresponds to the degree of double refraction. The optical axis refers to a direction of a ray when the above sectioned plane becomes a circle. In this case, no double refraction will occur.
If the substrate is of a uniaxial crystal, the above sectioned plane becomes a circle only in one direction. If the substrate is of a biaxial crystal, the sectioned plane becomes a circle, in two directions. These two directions are indicated by A.sub.1, A.sub.2 in FIG. 1. In the plane including A.sub.1, A.sub.2, the direction in which the angle between A.sub.1 and A.sub.2 is divided into two equal angles .theta..sub.A represents a direction of one main refractive index N.sub.3 among the three main refractive indices. The directions of the remaining two main refractive indices N.sub.1, N.sub.2 lie in the planes perpendicular to the direction of N.sub.3.
For a PC (polycarbonate) plate injection-molded for the optical disc, the magnitudes of N.sub.1 and N.sub.2 are almost equal to each other, while the magnitude of N.sub.3 is larger than the magnitudes of N.sub.1 and N.sub.2. The resulting index ellipsoid has a shape of a Rugby ball. Furthermore, N.sub.3 shows optical anisotropy substantially perpendicular to the surface of the disc substrate. The fact mentioned above is already known is described in the following paper by the inventor: Akihiko Yoshizawa, "An Analysis of Optical Anisotropy of PC Substrate for Magneto-Optical Disc", Optics, Vol. 15, No. 3 (1986), referred to as Reference 1 hereinafter.
For the reason, the status of the index ellipsoid is not known at all in the conventional method of measuring double refraction with a normal incident ray. In an actual optical pick-up using condensed rays with different incident angles, therefore, how the degree of double refraction will affect a reproduced signal cannot be determined.
Generally, the directions of the three main refractive indices, i.e., the mains axes of the index ellipsoid, and the magnitudes or lengths thereof have to be known in order to determine the status of the index ellipsoid. For a the substrate of the optical disc, it sometimes happen that while the directions of the main axes are almost known, their magnitudes are a problem to determine. One method of determining those magnitudes is disclosed in the above cited U.S. Pat. No. 4,792,227. This method is also explained in detail in the above cited Reference 1 by the inventor. Because this method basically relies on a magneto-optical head actually employed, the plane of polarization of an incident linearly polarized light is fixed to be normal to the radius of the disc, and hence the angle between the incident plane and the polarized plane is varied upon changes of an azimuth. (In contrast, the angle between the incident plane and the polarized plane will be fixed to a certain angle in this embodiment.) While a uniaxial model has been supposed in the above cited U.S. patent, a biaxial model is required for more precise measurement.
Therefore, the theoretical formulae will be rewritten below so as to be match with the biaxial model.